Abstract
The Bi2O3 based ZnO varistors (MOVs) are used in surge arresters or surge protective devices (SPDs) as key components. As their performance, MOVs shall have I-V non ohmic characteristics, higher surge withstand capability and reliability for ageing: When AC power line voltage is applied to MOV, its leakage current or watt loss shall be decreased continuously.
In order to maintain reliability for ageing, it is known that additional heat treatment after sintering process in MOV manufacturing flow is effective. The mechanism of the heat treatment effect was considered to be realized by a crystal structure change of the bismuth oxide as a main material which forms a grain boundary. However, in this study, it is found that watt loss behavior does not necessarily correspond to crystal structure of the bismuth oxide. Besides, from the fact that ZnO which dissolve in Bi2O3 liquid phase is compounded with other additives by heat treatment, precipitation of ZnO in Bi2O3 liquid phase by heat treatment can be considered as important factor.
In addition, uniformity not only in grain boundary structure from a microscopic of view, but also of size and its distribution of the multiple points from a macroscopic of view is integrant in order to realize MOVs having lower watt loss and its stability.
